This invention concerns a method and apparatus for generating a low-sulfur reducing gas by the gasification of carbonaceous fuel in a molten iron bath. Fuel, oxygen and a slag former are injected into the molten metal bath through the bottom of the bath-containing vessel. The reaction within the vessel is cooled by the injection of a gaseous coolant which is recycled spent top gas from a direct reduction shaft furnace, steam or a mixture thereof.
The present invention is an improvement to German OLS 27 50 275, which teaches the generation of a reducing gas in a molten metal bath. The known gasifier is noted for its refractory wear. The present invention alleviates the refractory wear problem by maintaining the bath temperature of the gasifier at a lower level. This is achieved by injecting a coolant into the molten metal bath to cool the reaction.
Sanzenbacher et al. U.S. Pat. No. 4,238,226 and Scarlett et al. U.S. Pat. No. 4,248,626 both describe methods for producing gas for direct reduction and the melting of direct reduced iron in a chamber containing a molten metal bath. No coolant is used in either of these processes because the melting of the direct reduced iron maintains the temperature of the molten metal bath in the desired range. A portion of Sanzenbacher's cooled gas from the cyclone provides compressed gas as a carrier for injection of coal into the bath, which compressed gas is not capable of providing sufficient cooling capacity to control the temperature of the bath, as the amount of carrier gas required to inject coal is too small to provide the needed coolant. Scarlett et al teaches injection of a humidified cooling gas above the melt, which has no effect on the melt.
Nemeth U.S. Pat. No. 3,853,538 teaches the use of a conventional entrainment gasifier which generates a molten slag from ungasified fuel and ash. Since there is no iron in the Nemeth gasifier, any cooling that would take place in the gasifier would clearly not take place in a molten iron bath.
Geskin U.S. Pat. No. 4,422,872 teaches a method for producing a gaseous fuel and heat from a melt by injecting air, coal, and steam into a melt. The heat evolved is abstracted from the melt by injection of heated gases into molten slag, rather than by injecting cooled, CO.sub.2 -lean off-gases from a direct reduction furnace into a molten iron bath.
British Patent Application No. 2,078,779A teaches that waste gases may be used to transport carbon-carrying material to the nozzles and/or to cool the nozzles, which is not analogous to cooling of the melt.
Although a carrier gas is used to inject coal or a carbon containing medium into the melt in some of the above-mentioned references, the amount of this carrier gas is so insignificant that it is inadequate to perform any appreciable cooling or even to have any real effect on the process.
The present invention also provides a well-balanced reducing gas for the direct reduction of iron. This is achieved by maintaining the operating temperature of the molten bath gasifier above the iron-carbon eutectic point.
In the direct reduction of iron oxide to metallized iron in a shaft furnace, the reacted top gas is superheated and must be cooled immediately upon removal from the furnace. The present invention utilizes this superheat to calcine lime for sulfur removal from the shaft furnace.